Transesterification, or the exchange reaction of esters with alcohols (an alcoholysis reaction), is an important class of reactions which may be catalyzed by both acid and base catalysts. Examples of transesterification, in general, include chemical reactions involving organic carbonates and carboxylic acid esters as reactants, products, or both. Other transesterification reactions include the production of biodiesel by transesterification of triglycerides with ethanol or methanol. Alcoholysis, in general, is a reaction where one or more functional groups of a compound are replaced by alkoxy or aryloxy group of an alcohol (alkyl or aryl hydroxyl compound). Examples of alcoholysis include chemical reactions involving urea, where amine groups are replaced by alkoxy groups to produce organic carbamates and carbonates.
Carboxylic acid esters are produced by transesterification of a carboxylic acid ester with an alcohol in the presence of acid and base catalyst. Sulfuric acid (homogeneous) and acid resins (solid) are preferred acid catalysts. Soluble bases, such as NaOH and KOH, various Na/K alkoxides or amines (homogeneous), and various basic resins (solid) are preferred base catalysts. Although catalysts can be either homogeneous catalyst or heterogeneous catalyst for the transesterification of carboxylic esters, base catalysts are, in general, more effective than acid catalysts. For example, long chain alkyl methacrylic esters are produced by exchange reaction of methyl methacrylate with a long chain alcohol in the presence of a base catalyst.
Biodiesel may be produced by transesterification of vegetable oils (triglycerides) with methanol or ethanol by using a homogeneous base catalyst, such as sodium methoxide or calcium acetate, as disclosed in U.S. Pat. Nos. 6,712,867 and 5,525,126, and a basic solid catalyst, such as a mixed oxide of zinc oxide and alumina or zinc aluminate (zinc oxide supported on alumina and calcined at a high temperature). Solid zinc aluminate catalysts are disclosed in U.S. Pat. No. 5,908,946 and U.S. Patent Application Publication No. 2004/0034244, for example.
U.S. Pat. No. 5,908,946 discloses a two-step process for producing esters by reacting vegetable oils or animal oils with an alcohol in the presence of solid catalysts such as zinc oxide or spinel type zinc aluminates. In the first step, the conversion of triglyceride is forced to a high conversion, usually higher than 90%. In the second step, the remaining triglycerides, diglycerides and monoglycerides are converted. The transesterifications are performed at a temperature from 230 to 245° C. at about 5.2 bar (about 725 psia). High conversion requires relatively low flow rates of a feed mixture (0.5 h−1 or lower space velocity).
U.S. Pat. No. 6,147,196 discloses a process for producing high purity fatty acid esters from plant or animal oil in the presence of a heterogeneous catalyst (zinc aluminate). U.S. Patent Application Publication No. 2004/0034244 relates to a processing scheme for producing alkyl esters from vegetable or animal oil and an alcohol in the presence of a heterogeneous catalyst (zinc aluminate). The esters are produced by transesterification in two fixed bed reactors. High conversion of triglyceride was obtained in the first reactor. After separating glycerol from the first transesterification reaction stream, the remaining unconverted triglyceride, diglyceride and monoglyceride are converted to esters in the second reactor. The transesterification is performed at 200° C., about 62 bar (900 psia) and 0.5 h−1 space velocity.
W. Xie et al. (J. Mol. Cat. A: Chem. 246, 2006, pp 24-32) discuss methanolysis of soybean oil in the presence of a calcined Mg—Al hydrotalcite catalyst. The calcined hydrotalcites with an Mg/Al ratio of 3.0 derived from calcinations at 500° C. is a catalyst that can give high basicity and excellent catalytic activity for this reaction. They report the soluble basicity of the hydrotalcites calcined at various temperatures.
Diesel engines emit more particulates and NOx than gasoline engines. It is reported that dialkyl carbonates are effective in reducing particulates in diesel engine exhaust. According to U.S. Pat. No. 5,954,280, urea and ammonia are effective NOx reducing agents. But using urea and ammonia for diesel engine has practical problems or inconveniences. U.S. Pat. No. 6,017,368 discloses ethyl carbamate as effective at reducing NOx from diesel engines. U.S. Pat. No. 4,731,231 (1988) reports that sublimed cyanuric acid can be an effective agent for the elimination or reduction of NOx. High temperature sublimation of cyanuric acid produces isocyanic acid (HNCO), which is believed to be responsible for elimination of NOx. EP 0363681 and EP 0636681 disclose a carbonate ester of an aliphatic triol or tetraol as a component of low smoke lubricating agents.
N-aryl methyl carbamate is produced by reaction of an aromatic amine with a dimethyl carbonate, typically in the presence of a base catalyst due to low reaction rates in the absence of a catalyst. N-aryl methyl carbamate can be decomposed to produce aromatic isocyanate at elevated temperature. For example, toluene dicarbamate is produced by reacting toluene diamine with dimethyl carbonate in the presence of a catalyst. Decomposition of toluene dicarbamate at elevated temperature yields toluene diisocyanate.
Organic carbonates (diesters of carbonic acid) are useful compounds that may be used as solvents, alkylating agents, carbonylation agents, co-polymerization agents, fuel additives, etc. Dimethyl carbonate (DMC) is an important dialkyl carbonate, commonly used as a raw material for the production of diphenyl carbonate (DPC, a diaryl carbonate). There are various processes for commercial production of DMC. In one such commercial process, DMC is produced by transesterification of a cyclic carbonate with methanol in the presence of a homogeneous catalyst. Although patents may disclose use of homogeneous catalysts or heterogeneous catalysts for transesterification of a cyclic carbonate with methanol, there is currently no commercial practice where a heterogeneous or solid catalyst is used for the production of DMC, likely due to the short cycle length of heterogeneous catalysts for such processes. DPC is commonly co-polymerized with a diol, such as bisphenol A, to produce polycarbonates. Polycarbonates are used in various special applications such as memory disks, windshields, engineering plastics, optical materials, etc.
Current techniques for production of diaryl carbonates using a non-phosgene process produces aromatic carbonates, such as DPC, by transesterification of DMC with phenol to produce methyl phenyl carbonate and methanol, followed by disproportionation of the methyl phenyl carbonate to produce DPC and DMC in the presence of homogeneous organometallic catalysts by employing a series of multiple reactive distillation reactors. A preferred homogeneous catalyst is titanium alkoxide. Such processes are disclosed in U.S. Pat. Nos. 4,045,464, 4,554,110, 5,210,268, and 6,093,842, for example. The homogeneous catalysts are recovered from the heaviest portion of the product streams as a solid, which may then be converted to soluble homogeneous catalyst to recycle.
Use of a homogeneous catalyst in the production of DPC often requires separation of the homogeneous catalyst from the product, especially where the catalysts are used at relatively high feed rates. To alleviate this and other shortcomings associated with using homogeneous catalysts for the production of diaryl carbonates, U.S. Pat. Nos. 5,354,923 and 5,565,605, and PCT Application Publication WO03/066569 disclose alternative processes where heterogeneous catalysts are used. For example, U.S. Pat. No. 5,354,923 discloses titanium oxide catalysts in powder form to demonstrate the preparation of EPC, MPC and DPC from DEC or DMC and phenol. U.S. Pat. No. 5,565,605 discloses microporous materials containing Group 4 elements as the catalysts for transesterification and disproportionation. However, solid catalysts in powder form are typically unsuitable or less preferable for large volume commercial production of DPC or methyl phenyl carbonate. WO03/066569 discloses a process for continuously producing DPC in the presence of a heterogeneous catalyst prepared by supporting titanium oxide on silica in a two-step fixed bed process by reacting DMC with phenol.
Z-H Fu and Y. Ono (J. Mol. Catal. A. Chemical, 118 (1997), pp. 293-299) and JP Application No. HEI 07-6682 disclose heterogeneous catalysts for the preparation of diphenyl carbonate by transesterification of DMC with phenol to MPC and disproportionation of MPC to DPC in the presence of MoO3 or V2O5 supported on an inorganic support such as silica, zirconia, or titania. The transesterification and disproportion are carried out in a reactor-distillation tower consisting of a reactor and distillation tower with removal of the co-products by distillation.
U.S. Patent Application Publication Nos. 2007/0093672 ('672) (now U.S. Pat. No. 7,378,540) and 2007/0112214 ('214) (now U.S. Pat. No. 7,288,668) disclose processes for producing various organic carbonates, such as diaryl carbonates, including DPC, in the presence of heterogeneous catalysts. In the '214 publication, the necessary reactions (transesterification and disproportionation) are performed in liquid phase in the presence of a heterogeneous catalyst. Multiple fixed bed reactors for the transesterification and disproportionation reactions are connected to a single distillation column, where light compounds such as ethanol and DEC are removed as an overheads fraction, and the higher boiling compounds, including DPC, are removed as a mixed bottoms fraction. DPC is then recovered from the bottoms fraction.
The '672 publication discloses a process for making diaryl carbonates and dialkyl carbonates by performing the necessary reactions in a dual-phase (vapor and liquid) reaction over various solid catalysts for transesterification and disproportionation. The chemical reactions producing organic carbonates are performed in a series of fixed bed reactors, while performing separation of light co-product in liquid phase to vapor phase in order to shift the unfavorable equilibrium reaction toward the desired product. The process is especially useful for the production of alkyl aryl carbonates such as EPC (ethyl phenyl carbonate) and diaryl carbonates such as DPC (diphenyl carbonate). The process is also useful for the production of dialkyl carbonates such as DEC. A series of fixed bed reactors are connected at different positions on a single distillation column via side-draw streams and return streams. The distillation column also contains separation stages above the last reactor in the series and below the first reactor in the series. The heterogeneous catalysts may be prepared by depositing one or two metal oxides of Ti, Zr, Nb, Hf, Ta, Mo, V, Sb, etc. on porous supports, such as silica gel. The heterogeneous catalysts may also be prepared by grafting one or more organometallic compounds from the elements of Ti, Zr, Nb, Hf, Ta, Mo, V, Sb, etc. on a porous support, which has surface hydroxyl groups or a mixture of hydroxyl and alkoxy groups.
Various other processes for the production of organic carbonates with heterogeneous catalysts are disclosed in U.S. Pat. Nos. 5,231,212, 5,498,743, and 6,930,195.
P. Ball et al. (C1 Mol. Chem. Vol. 1, 1984, pp. 95-108) studied the chemistry of dialkyl carbonate production in the presence of various homogeneous or heterogeneous catalysts. For example, dimethyl carbonate is produced by alcoholysis of urea. Dibutyltin dimethoxide is reported as a particularly effective catalyst. It is reported that heterogeneous catalysts are also effective for the chemistry in the presence of co-catalysts, such as 4-dimethylaminopyridine and PPh3. The reported heterogeneous catalysts are Al2O3, Sb2O3, and silica. Fused SiO2 is not a catalyst, but becomes catalytic in the presence of PPh3.
In U.S. Pat. No. 7,074,951, dialkyl carbonate is produced by alcoholysis of urea with an alcohol in the presence of a homogeneous tin complex catalyst in the presence of a high boiling electron donor atom containing solvent, such as triglyme. This patent also demonstrates the capability of producing DMC continuously for about 1500 hours.
EP 1629888 and D. Wang et al. (Fuel Processing Tech. 88, 8, 2007, pp 807-812) disclose that DMC and DEC may be produced in the presence of zinc oxide and zinc oxide supported on silica. These publications are completely silent about the catalyst stability or catalyst cycle length.
Catalyst deactivation during transesterification and disproportionation reactions may be caused by the deposition of heavy polymers on the catalyst surface and pores. The catalyst deactivation rate by polymer deposition increases with the concentration of alkyl aryl carbonate and diaryl carbonate or both in a reaction mixture. Depolymerization of polymers on the heterogeneous catalysts is disclosed in the '672 publication. However, depolymerization may results in only a partial recovery of solid catalyst activity.
U.S. Pat. Nos. 6,768,020 and 6,835,858 disclose processes for making dialkyl carbonates and co-product propylene glycol by reaction of propylene carbonate with DMC, water, or both, in the presence of solid catalyst such as lanthanum oxide and zinc oxide supported on alumina, silica, etc. Catalyst instability is partially solved in U.S. Pat. No. 6,768,020 by depositing a large amount of lanthanum oxide on a support such as alumina and silica.
A favored technique to compensate for catalyst deactivation is the ramping up of the reaction temperature as the catalyst deactivates. This technique, unfortunately, often accelerates deactivation of heterogeneous catalysts.
Long, stable performance of a solid catalyst is generally required for commercial production using a heterogeneous catalyst. Catalyst costs, downtime associated with catalyst replacement, and other factors as known in the art dictate that heterogeneous catalysts have a minimum lifespan, typically greater than 3 months, 6 months, or a year, depending upon the process.
Although heterogeneous catalysis of various transesterification reactions is possible, as described by the various patents and publications above, they do not report longevity or cycle length of the catalyst. It is the experience of the present inventor that such heterogeneous catalysts have undesirably short cycle lengths.
Accordingly, there exists a need for transesterification and/or disproportionation processes using heterogeneous catalysts with improved catalyst performance.